A device for the treatment of exhaust gases is provided, such as a catalytic converter or a diesel particulate trap having a fragile structure mounted within a housing which is supported therein by a mounting mat disposed between the housing and the fragile structure.
A catalytic converter assembly for treating exhaust gases of an automotive or diesel engine contains a fragile structure, such as a catalyst support structure, for holding the catalyst that is used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen present in the exhaust gases. The fragile catalyst support structure is mounted within a metal housing, and is preferably made of a frangible material, such as a monolithic structure formed of metal or a brittle, fireproof ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure with a plurality of tiny flow channels. However, as noted hereinabove, these structures can be, and oftentimes are, very fragile. In fact, these monolithic structures can be so fragile that small shockloads or stresses are often sufficient to crack or crush them.
The fragile structure is contained within a metal housing, with a space or gap between the external surface of the fragile structure and the internal surface of the housing. In order to protect the fragile structure from thermal and mechanical shock and other stresses noted above, as well as to provide thermal insulation and a gas seal, and to hold the fragile catalyst support structure in place within the housing, it is known to position at least one ply or layer of mounting or support material within the gap between the fragile structure and the housing.
Presently, materials used in mounting mats for catalytic converters and other to exhaust gas-treating devices may range from relatively inexpensive materials such as, for example, amorphous glass fibers such as S-glass, to more expensive materials such as, for example, high alumina-containing ceramic oxide fibers. Intumescent materials as well as non-intumescent materials have been and continue to be employed in mounting mats, depending upon the application and conditions under which the mounting mats are to be used.
The type of monolithic structure to be employed as well as the application and the conditions under which the mounting mats are to be used must be determined prior to selection of the mounting mat materials. For example, one would use a high temperature resistant mounting mat material adaptable over a wide temperature range for a high temperature application such as typically found in catalytic converters, while a lower temperature resistant, resilient, flexible material may be just as or more suitable for high G load applications that use heavier substrates such as may be found in diesel catalyst structures and diesel particulate traps.
In any event, the mounting mat materials employed should be capable of satisfying any of a number of design or physical requirements set forth by the fragile structure manufacturers or the catalytic converter manufacturers. For example, a state-of-the-art ply or plies of mounting mat material, should preferably exert an effective residual holding pressure on the fragile structure, even when the catalytic converter has undergone wide temperature fluctuations, which causes significant expansion and contraction of the metal housing in relation to the fragile structure, also referred to as the catalyst support structure, which in turn causes significant compression and release cycles for the mounting mats over a period of time. The best, state-of-the-art mounting mats used in high temperature applications have been found to sufficiently hold the fragile structure in the most severe of applications where temperatures reach well above 900° C., and often undergo constant thermal cycling to room temperature.
Other mounting mats, while not requiring use in high temperature environments, must provide sufficient resiliency and flexibility to effectively hold the fragile structure with sufficient force or strength, but yet not crush the fragile structure under constant thermal cycling. Under normal operating conditions for a catalytic converter, a minimum shear strength for a mounting mat of at least 5 kPa is required to prevent the fragile structure from being dislodged and damaged. The shear strength of the mat is defined as the mat's holding pressure multiplied by the coefficient of friction of the mat/fragile structure interface. The coefficient of friction of typical mat products in catalytic converters is approximately 0.45 in the in-use condition. Therefore, a mounting mat for high temperature applications, i.e., those applications where the temperature in the catalytic converter may increase up to about 900° C. or more, should have an effective residual minimum holding pressure after 1000 cycles of testing at a hot face temperature of about 900° C. of at least about 10 kPa.
For other exhaust gas-treating devices, such as diesel particulate traps or diesel catalyst structures, it will be appreciated that while these devices do not reach the temperatures provided in high temperature catalytic converters, the weight of the fragile structure and loading techniques employed require the mounting mat utilized to have a different effective residual minimum holding pressure than that set forth above. In these applications, a higher minimum shear strength for the mounting mat of at least about 25 kPa is preferably achieved to prevent that fragile structure from being dislodged and damaged. The coefficient of friction of these mat products in such high G-load applications with heavy substrates is still approximately 0.45 in the in-use condition. Therefore, a mounting mat for this type of application should have an effective residual minimum holding pressure after 1000 cycles of testing at a temperature of about 300° C. of at least about 50 kPa.
Many mounting mats, heretofore, have attempted to overcome thermal cycling problems associated with high temperature applications by using high alumina or mullite ceramic fibers. In one known embodiment, an aqueous solution or a colloidal dispersion, often called an “organosol” or a “sol gel” is used to produce the ceramic fibers. While ceramic fibers formed by sol gel processes may offer a high degree of resiliency needed for mounting monolithic structures, the high cost of the fibers have forced manufacturers to seek other, less expensive solutions. In addition, these ceramic fibers typically have an average fiber diameter of less than 5, and in some cases, less than 3.5, microns. Thus, these fibers are respirable, i.e., capable of being breathed into the lungs.
In other instances, a fibrous mounting material may be used in combination with other materials, such as intumescent materials and backing layers, in order to provide sufficient strength for handleability, resiliency, or to obtain an adequate holding pressure.
As another alternative to using sol gel-derived, ceramic fibers, attempts have been made to form refractory ceramic fibers using melt-processing techniques. Only in the last ten years or so have refractory ceramic fibers, i.e., fibers containing from about 45 to 60 percent alumina and from about 40 to about 55 percent silica, satisfied manufacturers of high temperature catalytic converters by providing mounting mats having sufficient resiliency values to meet the manufacturers' demands. Not only are mounting mats containing such refractory ceramic fibers expensive, but also they are difficult to manufacture, particularly with respect to the processing treatments they must undergo. Care must be taken to ensure that they are substantially shot free.
In low temperature catalytic converter applications, such as turbocharged direct injection (TDI) diesel powered vehicles, the exhaust temperature is typically about 150° C. and may never exceed 300° C. Various types of mounting mats may be used for these and other slightly higher temperature applications. For many catalytic converter applications, intumescent mats, i.e., mounting mats made from intumescent materials such as graphite or vermiculite, have been used. It has more recently been observed that mounting mats made with intumescent materials may fail for these low temperature applications.
One possible reason for this failure is that the exhaust temperature may be too low to expand the intumescent, typically vermiculite, particles sufficiently. Thus, the mats fail to provide sufficient pressure against the fragile structure and tend to fail. A second possible reason for this failure is that organic binder systems used in the intumescent mat products degrade and cause a loss in the holding pressure.
Thus, non-intumescent mounting mat systems have been developed and are now common within the industry. These materials are suitable for use over a much wider temperature range than are the intumescent prior art mats.
Non-intumescent mat systems include substantially no intumescent materials such as graphite or vermiculite and, therefore, are substantially non-expanding. By “substantially non-expanding,” it is meant that the mat does not readily expand upon the application of heat as would be expected with intumescent mats. Of course, some expansion of the mat does occur based upon its thermal coefficient of expansion, but the amount of expansion is insubstantial and de minimus as compared to the expansion of mats employing useful amounts of intumescent material. These non-intumescent mats, heretofore, have comprised high temperature resistant, inorganic fibers and, optionally, a binder. By high temperature resistant, it is meant that the fiber can have a use temperature up to about 900° C. or greater. Depending upon the application, the temperature regime in which the mat is used, and the type of monolith employed, non-intumescent mats have, heretofore, been known to generally contain one or more types of fibers selected from alumina/silica fibers (available under the trademark FIBERFRAX from Unifrax Corporation, Niagara Falls, N.Y.) and high alumina fiber mats available from Saffil.
Presently, fibers employed in state-of-the-art non-intumescent mounting mats for higher temperature applications are generally high in alumina content. For example, refractory ceramic fibers are composed substantially of alumina and silica and typically contain from about 45 to about 60 percent by weight alumina and from about 40 to about 55 percent by weight silica, while other alumina/silica ceramic fibers, such as alumina or mullite ceramic fibers made by sol gel processing, usually contain more than 50 percent alumina. S2-glass fibers typically contain from about 64 to about 66 percent silica, from about 24 to about 25 percent alumina, and from about 9 to about 10 percent magnesia. Generally, it has been thought that the higher the amount of alumina employed in the fibers, the higher temperature for the application in which the fibers could be employed. The use of fibers consisting substantially of alumina have therefore been proposed for this purpose.
Other non-intumescent mounting mats are generally very thick and lack the structural integrity needed for the exhaust gas treatment device application, and may even require being handled in a bag to prevent crumbling of the mounting mat. These mounting mats are also difficult to cut to size for installation, and further must be compressed substantially to fit enough material needed for supportive mounting within the gap between the catalyst support structure and the housing.
Attempts have been made to use still other types of materials in the production of non-intumescent mounting mats for catalytic converters and other exhaust gas-treating devices for high temperature applications, such as flexible, nonwoven mounting mats comprising shot-free ceramic oxide fibers comprising aluminosilicate fibers containing from about 60 to about 85 percent by weight alumina and from about 40 to about 15 percent by weight silica; crystalline quartz fibers; or both. These aluminosilicate fibers have a higher alumina content than refractory ceramic fibers, but are produced using the sol gel techniques discussed above.
On the other hand, crystalline quartz fibers are made of essentially pure silica (i.e., 99.9 percent silica). These fibers are made by a melt drawing process using raw materials derived from crystalline quartz, and are not leached in any manner. Such fibers are available from J. P. Stevens, Slater, N.Y., under the tradename ASTROQUARTZ, or from Saint Gobain, Louisville, Ky., under the tradename QUARTZEL. However, the cost of these quartz fibers make them commercially prohibitive for use in mounting mats.
Similarly, U.S. Pat. No. 5,290,522 discloses a nonwoven mounting mat for a catalytic converter that may contain magnesia/alumina/silicate fibers such as is known in the art and commercially available from Owens Corning, Toledo, Ohio as S2-GLASS, as well as the ASTROQUARTZ quartz fibers discussed in the above cited patent. In this patent, it is expressly noted in Comparative Example 1 that a mounting mat containing a commercially available leached glass fiber containing silica did not pass the hot shake test used by the patentees to determine suitability as a mounting mat for higher temperature catalytic converters.
Mounting mats containing silica fibers in combination with intumescent materials have been tested for catalytic converter use, for example in German Patent Publication No. 19858025.
A detailed description and process for making leached glass fibers high in silica content is contained in U.S. Pat. No. 2,624,658, the entire disclosure of which is incorporated herein by reference. Another process for making leached glass fibers high in silica content is disclosed in European Patent Application Publication No. 0973697. While both the U.S. patent and the European patent application publication disclose the production of leached silica fibers in the formation of high temperature resistant products made from the resultant fibers, there is no mention whatsoever of the fibers being suitable for use or even being capable of being used as mounting mats for exhaust gas treatment devices, such as catalytic converters.